The invention relates to a process for producing submicron-sized particles of polymer encapsulted solid pigments and similar solid active ingredients, wherein a polymer matrix completely surrounds the active ingredient and the resulting particles are configured to have a buoyancy which is either neutral in an aqueous suspension, or chosen for a particular relationship to another ultimate carrier fluid. The present encapsulation technique is disclosed to have three specific applications, in the field of low viscosity aqueous colorants. Historically, the application of a given solid colorants to a substrate, such as paper, textile, or human skin, has required a unique dispersion chemistry.
The first application is production of low viscosity, aqueous writing fluids, wherein the active ingredient comprises a pigment comprised of primary particles in the submicron range, wherein the process is controlled to produce a neutrally buoyant suspension of encapsulated particles within water.
The second application relates to the problem of textile coloration wherein there is need to modify intrinsic characteristics of pigments in order to create a more predictable or uniform action of the colorant upon various textile substrates.
The third application relates to the preparation of cosmetics, wherein a solid active ingredient is modified and a composition is produced which is either film-forming, or non-film-forming, whereby the polymer matrix is adjusted to provide skin interaction characteristics that are independent of those inherent in the encapsulated solid ingredient.
The present invention categorically involves handling of solid particulate matter, in the range 0.001 to approximately 2.0 microns in average diameter. Solids categorized as pigments that have primary particles processed to be submicron in average size are a preferred starting material. In addition, to small primary particle size, the particles need to be dispersed homogeneously within a particular class of solvents and polymers, without any appreciable agglomeration. The present invention also categorically begins with a polymer that substantially is insoluble in water, and does not undergo chemical change to achieve an encapsulated particle. If the polymer substantially retains its physical characteristics during the process, the performance and rheology of a resulting fluid substantially is predictable, without critical change upon substitutions of other solid active ingredients, having similar size and surface characteristics.
The present invention categorically is not an emulsion polymerization, since the primary particles are not dispersed in a monomer or any mixture of monomers, with a polymerization subsequent to the emulsification step. Representative prior art techniques for producing colloidal size hydrophobic polymer particulates that surround discrete particles of inorganic material are discussed by SOLC nee Hajna, (U.S. Pat. No. 4,421,660), which teaches an emulsification polymerization for surrounding discrete primary particles with a polymer matrix, which then are useful for applications similar to the applications taught herein. The present invention categorically avoids need for adjusting conditions of emulsification so as to induce polymerization of a monomer, although the present invention particularly requires a solvent in an intermediate step which must be removed, requiring energy to produce the end product. Unlike suspension polymerization, however, the present invention is capable of encapsulating particles or molecules at very wide concentration ranges, molecule to polymer ratios of from 1/10 to 10/1, by weight, and also capable of employing a wide variety of polymer candidates having widely varying physical properties, under virtually identical process conditions. In other words, since there is no need to establish hospitable conditions for a polymerization of monomer, the present invention requires matching only of physical characteristics of a polymer through the solvent and water components. The choice of surfactants also is greatly simplified since surfactants are known to interfere with polymerization reactions. Emulsion polymerization requires satisfaction of many different variables, which limit the nature and concentration of materials which successfully may be encapsulated. The complexity of emulsion polymerization further requires a new investigation of appropriate conditions to induce a polymerization for a desired concentration and material for each specific application. Another dispersion polymerization technique employing solid particulate materials is represented by Bayley (U.S. Pat. No. 4,264,700) wherein toner particles are prepared by a suspension polymerization, and both a monomer and a solvent are included for preparation of a discontinuous phase. The monomer is emulsified in order to allow the monomer to migrate through the aqueous medium, and polymerize at active sites on the solid toner particles. It also is known from Nguyen et al. (U.S. Pat. No. 4,530,961) how to create an aqueous dispersion of carbon black for an ink jet writing fluid, by grafting active sites on carbon particles to chains of monomer units. Here again, a simultaneous polymerization and grafting is required.
Removal of the intermediate organic solvent is a critical aspect of the present invention, since a separation or coagulation of the system during an evaporation or distillation step will destroy the desired result. There has existed a need to avoid the unpredictable interactions inherent in dispersion polymerization, so that off-the-shelf polymer candidates can be chosen for their intrinsic physical properties, and substantially contribute those intrinsic properties to encapsulations of various solid particulate materials.